1. Field of the Invention
The invention relates to a driving circuit and a power converter incorporating the driving circuit, more particularly to an automatically self-correcting driving circuit and a power converter incorporating the same.
2. Description of the Related Art
As shown in FIG. 1, a conventional self-driven synchronous rectifying power converter includes a main transformer 71, a driving switch 72, a rectifying switch 73, a loop switch 74, an inductor 75, and a capacitor 76. The main transformer 71 includes a primary coil 711 and a secondary coil 712.
The primary coil 711 has a dotted terminal that is adapted to be coupled to an input power source 77, and an un-dotted terminal that is connected electrically to the driving switch 72. The driving switch 72 is driven at a control terminal thereof by a driving signal. An external load 78 is adapted to be coupled in parallel to the capacitor 76.
When the driving switch 72 is turned on by the driving signal, a transfer current is induced in the secondary coil 712 by the primary coil 711 as the input power source 77 supplies input power to the primary coil 711. As a result, the rectifying switch 73 is turned on. At this time, the loop switch 74 is turned off. The transfer current flows from a dotted terminal of the secondary coil 712 toward the inductor 75 so as to charge the capacitor 76 and to transfer the energy to the external load 78.
Subsequently, when the driving switch 72 is turned off by the driving signal, the rectifying switch 73 is turned off. At this time, the loop switch 74 is turned on, and the transfer current flows from the capacitor 76 to the loop switch 74 through the inductor 75 as the capacitor 76 discharges.
Until the driving switch 72 is turned on once again by the driving signal such that the rectifying switch 73 is turned on, while the loop switch 74 remains turned on, the transfer current flows from the capacitor 76 to the inductor 75 with decreasing magnitude. At this time, a portion of the transfer current flows from the inductor 75 to the secondary coil 712 through the loop switch 74 and the rectifying switch 73, such that a reverse current is induced in the primary coil 711 and eventually flows from the primary coil 711 toward the input power source 77. This phenomenon is known as the reverse current phenomenon.
Subsequently, when the magnitude of the transfer current flowing from the capacitor 76 to the inductor 75 becomes zero, the loop switch 74 is turned off, and the transfer current changes back to flowing from the secondary coil 712 to the inductor 75 such that the energy is transferred to the external load 78 again.
In the conventional self-driven synchronous rectifying power converter, upon switching of the driving switch 72 from being turned off to being turned on, the loop switch 74 is unable to switch immediately to being turned off, resulting in the reverse current phenomenon, thereby causing power loss and diminishing the power conversion efficiency.
As shown in FIG. 2, as disclosed in Taiwanese Invention Patent No. I220,084, a conventional synchronous rectifying power converter controlled by a current transformer is suitable for a non-continuous current operating mode. The conventional synchronous rectifying power converter includes a switch transistor 82, a flyback transformer 81, a current transformer 87, a control circuit 84, a switch driving unit 85, a synchronous rectifying switch 83, and a capacitor 86. The flyback transformer 81 includes a first coil 811 and a second coil 812. The current transformer 87 includes a third coil 871 and a fourth coil 872.
The first coil 811 has an un-dotted terminal adapted to be connected electrically to an input power source 88. The switch transistor 82 has a control terminal receiving a drive signal. The capacitor 86 is adapted to be coupled to an external load 89.
When the switch transistor 82 is turned on according to the drive signal received at the control terminal of the switch transistor 82, a voltage corresponding to an input power supplied by the input power source 88 is induced across the fourth coil 872 of the current transformer 87 via the flyback transformer 81 and the third coil 871. The control circuit 84 receives the induced voltage from the fourth coil 872 for controlling the switch driving unit 85 to determine conduction state of the synchronous rectifying switch 83, so as to thereby determine whether the energy is to be transmitted to the external load 89.
When an induced current flows from the capacitor 86 into the second coil 812, and when the switch transistor 82 is turned on, the conventional synchronous rectifying power converter is capable of ensuring that the synchronous rectifying switch 83 is turned off, such that the reverse current does not flow back into the input power source 88. However, the switch driving unit 85 is unable to synchronize control of the synchronous rectifying switch 83 with the drive signal.
A conventional way of solving the problems related to driving capability, reverse current and circuit synchronicity involves utilization of a driving chip. However, the driving chip is costly, thereby increasing the overall cost of the conventional power converters.